1. Field of the Invention
The present invention relates to a technology for scanning a target surface with a light beam.
2. Description of the Related Art
In recent years, there has been a growing demand for image forming apparatuses, such as optical printing apparatuses, digital copiers, and optical plotters, which are inexpensive and provide highly stable image quality regardless of temperature changes.
When the temperature changes, the oscillation wavelength of the light source or the shape of an optical element may change, and these changes may cause a shift in imaging position, which decreases the image quality.
The adoption of a diffractive optical element has been proposed as an effective solution to this problem (e.g., see Japanese Patent Application Laid-open No. 2002-287062 and Japanese Patent Application Laid-open No. 2007-241182). A diffractive optical element can be regarded as an optical element having negative dispersion. The entire scanning optical system is temperature compensated by endowing the diffractive optical element with diffraction power high enough to cancel out a shift in mage-forming position, thus enhancing the stability of image quality.
However, the laser-scanning apparatus disclosed in Japanese Patent Application Laid-open No. 2002-287062 has a problem in that manufacturing is difficult and expensive because a transmissive surface and a reflecting surface need to be formed and a curved reflecting surface also needs to be provided in one optical element. In addition, because this laser-scanning apparatus includes an optical element with a large height in the sub-scanning direction, if it is used, for example, in a so-called tandem color machine, the size of the machine will be increased.
On the other hand, for the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2007-241182, because a collimator lens with a multi-level diffraction surface is made of resin, the collimator lens itself has a large linear expansion coefficient of 7.0×10−5/K. This leads to a large phase function coefficient, that is, to large diffraction power, which causes the optical scanning device to be vulnerable to a change in wavelength.